Radio terminal, base station, and method

ABSTRACT

A base station, apparatus for controlling a base station, and method used in a base station comprises transmitting a resume identifier used for resuming an RRC connection of a user equipment, to the user equipment when causing the user equipment to transition from an RRC connected state to an RRC inactive state in which a paging area is configured in the user equipment from the base station, transmitting to another base station, a paging request message for paging the user equipment, the paging request including the resume identifier and information indicating a paging DRX (Discontinuous Reception) cycle of the user equipment, and receiving from the another base station, first information indicating that the another base station has failed in paging the user equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/521,022 filed Jul. 24, 2019, which is a continuation application ofinternational application PCT/JP2018/003617, filed Feb. 2, 2018, whichclaims the benefit of U.S. Provisional Application No. 62/454,177 filedFeb. 3, 2017, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a radio terminal, a base station, anda method used in a mobile communication system.

BACKGROUND ART

In recent years, with the spread of radio terminals such as smartphonesthat can execute a large number of applications, the frequency at whichradio terminals are connected to a network and the frequency at which anetwork performs paging of radio terminals have been increasing.

For this reason, in the mobile communication system, a network loadassociated with signaling is increasing. Under such circumstances, in3rd generation partnership project (3GPP) which is a standardizationproject of a mobile communication system, technology for reducingsignaling is under review.

SUMMARY

A base station according to an embodiment is used in a mobilecommunication system. The base station comprises a controller configuredto transmit a resume identifier used for resuming an RRC connection of auser equipment, to the user equipment when causing the user equipment totransition from an RRC connected state to an RRC inactive state in whicha paging area is configured in the user equipment from the base station,transmit to another base station, a paging request message for pagingthe user equipment, the paging request including the resume identifierand information indicating a paging DRX (Discontinuous Reception) cycleof the user equipment, and receive from the another base station, firstinformation indicating that the another base station has failed inpaging the user equipment.

An apparatus according to an embodiment for controlling a base stationcomprises a processor and a memory. The processor is configured to causethe base station to transmit a resume identifier used for resuming anRRC connection of a user equipment, to the user equipment when causingthe user equipment to transition from an RRC connected state to an RRCinactive state in which a paging area is configured in the userequipment from the base station, transmit to another base station, apaging request message for paging the user equipment, the paging requestincluding the resume identifier and information indicating a paging DRX(Discontinuous Reception) cycle of the user equipment, and receive fromthe another base station, first information indicating that the anotherbase station has failed in paging the user equipment.

A method according to an embodiment is used in a base station. Themethod comprises transmitting a resume identifier used for resuming anRRC connection of a user equipment, to the user equipment when causingthe user equipment to transition from an RRC connected state to an RRCinactive state in which a paging area is configured in the userequipment from the base station, transmitting, to another base station,a paging request message for paging the user equipment, the pagingrequest including the resume identifier and information indicating apaging DRX (Discontinuous Reception) cycle of the user equipment, andreceiving, from the another base station, first information indicatingthat the another base station has failed in paging the user equipment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an LTE systemaccording to an embodiment.

FIG. 2 is a diagram illustrating a configuration of a UE (radioterminal) according to an embodiment.

FIG. 3 is a diagram illustrating a configuration of an eNB (basestation) according to an embodiment.

FIG. 4 is a diagram illustrating a configuration of a protocol stack ofa radio interface in an LTE system according to an embodiment.

FIG. 5 is a diagram illustrating a configuration of a radio frame usedin an LTE system according to an embodiment.

FIG. 6 is a diagram illustrating an operation example of a UE accordingto an operation pattern 1 of a first embodiment.

FIG. 7 is a diagram illustrating an operation sequence example accordingto the operation pattern 1 of the first embodiment.

FIG. 8 is a diagram illustrating an operation example of a UE accordingto an operation pattern 2 of the first embodiment.

FIG. 9 is a diagram illustrating an assumed scenario according to asecond embodiment.

FIG. 10 is a diagram illustrating an operation example according to amodified example of a third embodiment.

FIG. 11 is a diagram illustrating an operation example according to afourth embodiment.

FIG. 12 is a diagram illustrating an operation example according to afirst modified example of the fourth embodiment.

FIGS. 13A to 13C are diagrams illustrating an operation exampleaccording to a fifth embodiment.

FIG. 14 is a diagram related to an additional note.

FIG. 15 is a diagram related to an additional note.

FIG. 16 is a diagram related to an additional note.

DESCRIPTION OF EMBODIMENTS

(Mobile Communication System) A configuration of a mobile communicationsystem according to an embodiment will be described. FIG. 1 is a diagramillustrating a configuration of a Long Term Evolution (LTE) system whichis a mobile communication system according to an embodiment. The LTEsystem is a mobile communication system based on the 3GPP standard.

The LTE system includes a radio terminal (user equipment (UE)) 100, aradio access network (evolved-UMTS terrestrial radio access network(E-UTRAN)) 10, and a core network (evolved packet core (EPC)) 20.

The UE 100 is a mobile communication apparatus. The UE 100 performsradio communication with an eNB 200 that manages a cell (serving cell)in which the UE 100 exists.

The E-UTRAN 10 includes base stations (evolved Node-Bs (eNBs)) 200. TheeNBs 200 are connected to each other via an X2 interface. The eNB 200manages one or more cells. The eNB 200 performs radio communication withthe UE 100 that has established a connection with its own cell. The eNB200 has a radio resource management (RRM) function, a routing functionof user data (hereinafter referred to simply as “data”), a measurementcontrol function for mobility control and scheduling, and the like. A“cell” is used as a term indicating the smallest unit of a radiocommunication area. The “cell” is also used as a term indicating afunction or resource for performing radio communication with the UE 100.

The EPC 20 includes a mobility management entity (MME) and a servinggateway (S-GW) 300. The MME performs various types of mobility controland the like on the UE 100. The MME manages information of a trackingarea (TA) in which the UE 100 exists by communicating with the UE 100using non-access stratum (NAS) signaling. The tracking area is an areaincluding a plurality of cells. The S-GW performs data transfer control.The MME and the S-GW are connected with the eNB 200 via an S1 interface.

FIG. 2 is a diagram illustrating the configuration of UE 100 (radioterminal). The UE 100 includes a receiver 110, a transmitter 120, and acontroller 130.

The receiver 110 performs various types of reception under the controlof the controller 130. The receiver 110 includes an antenna and areceiving section. The receiving section converts a radio signalreceived by the antenna into a baseband signal (reception signal) andoutputs it to the controller 130.

The transmitter 120 performs various types of transmission under thecontrol of the controller 130. The transmitter 120 includes an antennaand a transmitting section. The transmitting section converts a basebandsignal (transmission signal) output from the controller 130 into a radiosignal and transmits it from the antenna.

The controller 130 performs various types of controls in the UE 100. Thecontroller 130 includes at least one processor and a memory. The memorystores a program executed by the processor and information used in aprocess by the processor. The processor may include a baseband processorand a central processing unit (CPU). The baseband processor performsmodulation/demodulation, encoding/decoding, and the like of basebandsignals. The CPU executes the program stored in the memory to performvarious types of processes. The processor executes a process to bedescribed later.

FIG. 3 is a diagram illustrating a configuration of the eNB 200 (basestation). The eNB 200 includes a transmitter 210, a receiver 220, acontroller 230, and a backhaul communication unit 240.

The transmitter 210 performs various types of transmission under thecontrol of the controller 230. The transmitter 210 includes an antennaand a transmitting section. The transmitting section converts a basebandsignal (transmission signal) output from the controller 230 into a radiosignal and transmits it from the antenna.

The receiver 220 performs various types of reception under the controlof the controller 230. The receiver 220 includes an antenna and areceiving section. The receiving section converts a radio signalreceived by the antenna into a baseband signal (received signal) andoutputs it to the controller 230.

The controller 230 performs various types of controls in the eNB 200.The controller 230 includes at least one processor and a memory. Thememory stores a program executed by the processor and information usedin the process by the processor. The processor may include a basebandprocessor and a CPU. The baseband processor performsmodulation/demodulation, coding/decoding, and the like of basebandsignals. The CPU executes the program stored in the memory to performvarious types of processes. The processor executes a process to bedescribed later.

The backhaul communication unit 240 is connected to a neighbor eNB viathe X2 interface. The backhaul communication unit 240 is connected tothe MME/S-GW 300 via the S1 interface. The backhaul communication unit240 is used for communication performed on the X2 interface andcommunication performed on the S1 interface.

The MME includes a controller and a network communication unit. Thecontroller performs various types of controls in the MME. The controllerincludes at least one processor and memory. The memory stores a programexecuted by the processor and information used in the process by theprocessor. The processor may include a baseband processor and a CPU. Thebaseband processor performs modulation/demodulation, coding/decoding,and the like of baseband signals. The CPU executes the program stored inthe memory to perform various types of processes. The processor executesa process to be described later. The network communication unit isconnected to the eNB 200 via the S1 interface. The network communicationunit is used for communication performed on the S1 interface.

FIG. 4 is a diagram illustrating a configuration of a protocol stack ofa radio interface in an LTE system. As illustrated in FIG. 4, a radiointerface protocol is divided into first to third layers of an OSIreference model. The first layer is a physical (PHY) layer. The secondlayer includes a medium access control (MAC) layer, a radio link control(RLC) layer, and a packet data convergence protocol (PDCP) layer. Thethird layer contains a radio resource control (RRC) layer. The PHYlayer, the MAC layer, the RLC layer, the PDCP layer, and the RRC layerconstitute an access stratum (AS) layer.

The PHY layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Data and control information are transmitted between the PHYlayer of the UE 100 and the PHY layer of the eNB 200 via a physicalchannel.

The MAC layer performs data priority control, a retransmission processby hybrid ARQ (HARQ), a random-access procedure, and the like. Data andcontrol information are transmitted between the MAC layer of the UE 100and the MAC layer of the eNB 200 via a transport channel. The MAC layerof the eNB 200 includes a scheduler. The scheduler decides transportformats (transport block sizes and modulation and coding schemes (MCSs))of an uplink and a downlink, and resource blocks allocated to the UE100.

The RLC layer transmits data to the RLC layer on the reception sideusing the functions of the MAC layer and the PHY layer. Data and controlinformation are transmitted between the RLC layer of UE 100 and the RLClayer of eNB 200 via a logical channel.

The PDCP layer performs header compression/decompression andencryption/decryption.

The RRC layer is defined only in a control plane that handles controlinformation. RRC signaling for various types of settings is transmittedbetween the RRC layer of the UE 100 and the RRC layer of the eNB 200.The RRC layer controls the logical channel, the transport channel, andthe physical channel in response to establishment, re-establishment, andrelease of a radio bearer. When there is a connection (RRC connection)between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 isin an RRC connected mode. When there is no connection (RRC connection)between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 isin an RRC idle mode.

The NAS layer located above the RRC layer performs session management,mobility management, and the like. NAS signaling is transmitted betweenthe NAS layer of UE 100 and the NAS layer of an MME 300C. The UE 100 hasa function of an application layer or the like in addition to the radiointerface protocol.

FIG. 5 is a diagram illustrating a configuration of a radio frame usedin the LTE system. The radio frame includes ten sub frames on a timeaxis. Each sub frame includes two slots on the time axis. The length ofeach sub frame is 1 ms. The length of each slot is 0.5 ms. Each subframe includes a plurality of resource blocks (RBs) on a frequency axis.Each sub frame includes a plurality of symbols on the time axis. Eachresource block includes a plurality of sub carriers on the frequencyaxis. Specifically, an RB is constituted by 12 sub carriers and 1 slot.A resource element (RE) is constituted by one symbol and one subcarrier. Among radio resources (time and frequency resources) allocatedto the UE 100, frequency resources can be specified by resource blocks,and time resources can be specified by sub frames (or slots).

In downlink, an interval of several symbols in the head of each subframe is an area used as a physical downlink control channel (PDCCH) fortransmitting downlink control information mainly. The remaining part ofeach sub frame is an area which can be used as a physical downlinkshared channel (PDSCH) for transmitting downlink data mainly.

In uplink, both ends of each sub frame in the frequency direction areareas used as a physical uplink control channel (PUCCH) for transmittinguplink control information mainly. The remaining part of each sub frameis an area which can be used as a physical uplink shared channel (PUSCH)for transmitting uplink data mainly

(Specific State)

A specific state according to an embodiment will be described. Thespecific state is a state in which signaling for the UE 100 issuppressed while an S1 connection for the UE 100 is being maintained.The S1 connection is also referred to as an S1 bearer. The S1 connectionis a connection established between the eNB 200 and the EPC 20 on the S1interface. The S1 interface includes an S1-U interface for the userplane and an S1-MME interface for the control plane. The S1 connectionmay include an S1-U connection established between the eNB 200 and theS-GW on the S1-U interface and an S1-MME connection established betweenthe eNB 200 and the MME on the S1-C interface.

The specific state may be one state out of the RRC connected mode or onestate of the RRC idle mode. According to the specific state, signalingis reduced as compared to a general RRC connected mode. According to thespecific state, the UE 100 can start data communication more quicklythan in the general RRC idle mode. Hereinafter, the specific state isreferred to as a “Light Connection state (Light Connection sub state).”A case in which the specific state is one state of the RRC connectedmode is referred to as a “modeling A.” A case in which the specificstate is one state of the RRC idle mode is referred to as a “modelingB.”

RAN paging is applied to the UE 100 in the Light Connection state. TheRAN paging is performed in units of RAN paging areas in which paging iscontrolled by the E-UTRAN 10 (the eNB 200). The RAN paging area may bean area narrower than the tracking area. By introducing the RAN pagingarea, it is possible to reduce the number of cells in which paging isperformed for one UE 100. Therefore, it is possible to reduce signaling.As an example, the RAN paging area is constituted by a cell of an anchoreNB that maintains the S1 connection of the UE 100 in the LightConnection state and a cell of the eNB 200 nearby the anchor eNB. Thenearby eNB 200 may be an eNB 200 having an X2 interface with the anchoreNB. If the anchor eNB receives data or NAS signaling addressed to theUE 100 in the Light Connection state from the MME/S-GW 300, the anchoreNB may determine to perform RAN paging and perform paging of the UE 100together with the nearby eNB 200. The paging may be performed bytransmitting an RRC paging message.

A basic operation related to the Light Connection state will bedescribed below.

The UE 100 supporting the Light Connection state (Light Connectionoperation) gives a notification indicating the support for the LightConnection state (Light Connection operation) in UE capabilityinformation (UE-EUTRA-Capability).

The UE 100 transitions to the Light Connection state in accordance withRRC signaling. Specifically, the UE 100 is configured to the LightConnection state in accordance with unicast RRC signaling (an RRCreconfiguration message or an RRC release message).

The S1 connection of the UE 100 in the Light Connection state ismaintained in the “anchor eNB” and is active. The anchor eNB may be aneNB 200 which causes the UE 100 transition to the Light Connectionstate. When the UE 100 moves to another RAN paging area, the anchor eNBmay be switched.

The Light Connection state is an EPS Connection Management (ECM)connection state from a point of view of a network. The ECM indicatesthe connection state between the UE 100 and the core network (MME).

For UE 100 in the Light Connection state, paging (RAN paging) can beperformed by activation of the RAN (the E-UTRAN 10). The RAN paging maybe initiated by the anchor eNB. The RAN paging area is constituted byone cell or a plurality of cells. The plurality of cells may be managedby different eNBs. The paging message is defined by reusing the generalRRC paging message.

A paging process (RAN paging) is controlled by the anchor eNB.

The RAN paging area can be configured to be specific to the UE. TheUE-specific RAN paging area is configured in the UE 100 from the eNB 200by unicast signaling or broadcast signaling. The RAN paging area isdesignated by a cell list or a paging area ID. The RAN paging area maybe a single cell. The RAN paging area may be the same area as thetracking area.

The UE 100 in the Light Connection state executes a cell reselectionmechanism similar to the RRC idle mode.

Context information (UE AS context) of the UE 100 in the LightConnection state is maintained in both the UE and the anchor eNB.

When the UE 100 in the Light Connection state detects paging or startsdata transmission, the UE 100 recovers a connection with the eNB 200.Alternatively, the UE 100 may transition to the RRC connected mode.

The UE 100 in the Light Connection state gives a notification to thenetwork when the UE 100 moves out of the configured RAN paging area.

The UE 100 in the Light Connection state performs a DRX operation usinga parameter similar to a DRX operation in the RRC idle mode. A parameterfor deciding paging opportunities may include an ID of the UE (forexample, an IMSI, an S-TMSI, a Resume ID, or the like).

The UE 100 in the Light Connection state transitions to an operation ofthe general RRC connected mode in accordance with an RRC procedure. Inthe modeling A, the procedure is an RRC recovery (resume) procedure oran RRC reestablishment procedure. In the modeling B, the procedure is anRRC recovery (resume) procedure.

First Embodiment

A first embodiment will be described on the premise of the mobilecommunication system described above.

In the first embodiment, a scenario in which each eNB 200 (or each cell)transmits an identifier of the RAN paging area to which each eNB 200 (oreach cell) belongs through broadcast signaling is assumed. In such ascenario, a method in which the eNB 200 transmits the RAN paging areaidentifier configured in the UE 100 through unicast signaling may beconsidered.

However, such a method is likely to increase the processing load formanaging the RAN paging area identifier since the eNB 200 explicitlyconfigures the RAN paging area identifier in the UE 100. Also, the eNB200 may configure the identifier of the RAN paging area not includingthe cell in which the UE 100 exists in the UE 100. In this case, whenthe UE 100 is in the Light Connection state, it is necessary for the UE100 to give a notification to the network immediately because the UE 100is outside the configured RAN paging area. The first embodiment is anembodiment that makes it possible to solve such problems.

The UE 100 (the receiver 110) according to the first embodiment receivesunicast signaling that causes the UE 100 to transition to the LightConnection state from the eNB 200 via a serving cell. The UE 100 (thecontroller 130) transitions to the Light Connection state in response tothe reception of the unicast signaling. The Light Connection state is astate in which the RAN paging area identifier indicating the RAN pagingarea in which paging is managed by the RAN including the eNB 200 isconfigured in the UE 100. Even though the RAN paging area identifier isnot included in the unicast signaling, the UE 100 (the controller 130)holds an identifier of a predetermined area as the RAN paging areaidentifier configured in the UE 100 in the Light Connection state. Thepredetermined area is the serving cell or the RAN paging area to whichthe serving cell belongs.

According to the first embodiment, even though the eNB 200 does notexplicitly configure the RAN paging area identifier in the UE 100, theUE 100 recognizes the serving cell when the UE 100 transitions to theLight Connection state or the RAN paging area to which the serving cellbelongs as the RAN paging area configured in the UE 100 itself. That is,the serving cell when the UE 100 transitions to the Light Connectionstate or the RAN paging area to which the serving cell belongs isimplicitly configured as the RAN paging area applied to the UE 100.Accordingly, it is possible to solve the problem in the case in whichthe eNB 200 explicitly configures the RAN paging area identifier in theUE 100.

In an operation pattern 1 of the first embodiment, the UE 100 (thereceiver 110) receives, from the eNB 200, broadcast signaling includingthe identifier of the RAN paging area to which the serving cell belongs.The UE 100 (the controller 130) holds the identifier in the broadcastsignaling as the RAN paging area identifier configured in the UE 100 inthe Light Connection state.

In an operation pattern 2 of the first embodiment, unicast signalingindicating transition to the Light Connection state includes informationindicating that the RAN paging area configured in the UE 100 includesonly the current serving cell. The UE 100 (the controller 130) holds theidentifier of the serving cell as the RAN paging area identifierconfigured in the UE 100 in the Light Connection state. According to theoperation pattern 2, when the RAN paging area includes only one cell, itis possible to make it unnecessary to configure the RAN paging areaexplicitly in the UE 100. Further, when the cell identifier is used asthe RAN paging area identifier, it is possible to prevent the RAN pagingarea identifier from being exhausted. The cell identifier is, forexample, a physical cell ID (PCI) or an E-UTRAN cell global ID (ECGI).The PCI is specified on the basis of a synchronization signaltransmitted from the eNB 200. The ECGI is included in a SIB transmittedfrom the eNB 200.

In the first embodiment, the UE 100 (the controller 130) determines, inthe Light Connection state, whether or not the UE 100 has moved toanother cell not belonging to the RAN paging area (a predetermined area)configured in the UE 100. In response to the determination indicatingthat the UE 100 has moved to the other cell, the UE 100 (the controller130) transmits a notification to the other cell.

—Operation Pattern 1

FIG. 6 is a diagram illustrating an operation example of the UE 100according to the operation pattern 1 of the first embodiment.

In step S10, the UE 100 (the receiver 110) receives, from the servingcell, broadcast signaling including the identifier of the RAN pagingarea to which the serving cell belongs. The broadcast signaling may bebroadcast RRC signaling (system information block (SIB)). The UE 100(the controller 130) stores the identifier of the RAN paging area towhich the serving cell belongs.

In step S12, the UE 100 (the receiver 110) receives, from the servingcell, unicast signaling that causes transition to the Light Connectionstate. The unicast signaling may be unicast RRC signaling (the RRCreconfiguration message or the RRC release message).

In step S14, the UE 100 (the controller 130) transitions to the LightConnection state. Also, the UE 100 holds an identifier in the broadcastsignaling (that is, the RAN paging area identifier received in step S10)as the RAN paging area identifier configured in the UE 100.

As described above, when the UE 100 receives an instruction (the RRCConnection Reconfiguration or Release message) to transition to theLight Connection state, the UE 100 reads the RAN paging area identifierbeing currently broadcast or reads the RAN paging area identifier beingalready held if the RAN paging area identifier is valid and holds theRAN paging area identifier as the RAN paging area identifier allocatedto the UE 100 itself. The RAN paging area identifier may be stored in avariable held by the UE 100.

FIG. 7 is a diagram illustrating an operation sequence example accordingto the operation pattern 1 of the first embodiment.

In step S101, the eNB 200 transmits the SIB including the RAN pagingarea identifier (Paging Area ID) to which its own cell (or its own eNB)belongs. The UE 100 receives the SIB.

In step S102, the UE 100 stores the RAN paging area identifier (PagingArea ID) in the SIB.

In step S103, the eNB 200 transmits, to the UE 100, unicast signalingthat causes the UE 100 to transition to the Light Connection state(Instruction to enter LC). The UE 100 is in the RRC connected mode whenthe unicast signaling is received.

In step S104, the UE 100 transitions to the Light Connection state, andholds the identifier (that is, the RAN paging area identifier stored instep S102) in the SIB as the RAN paging area identifier configured inthe UE 100 itself. In other words, the UE 100 regards the RAN pagingarea identifier in the SIB as the RAN paging area identifier configuredin the UE 100 itself.

Thereafter, the UE 100 continues to stay in the cell in which the UE 100transitions to the Light Connection state or moves from the cell inwhich the UE 100 transitions to the Light Connection state to anothercell. Here, a case in which the UE 100 moves from the cell in which theUE 100 transitions to the Light Connection state to another cell ismainly assumed. The UE 100 reselects the other cell using a cellreselection mechanism similar to the RRC idle mode.

In step S105, the eNB 200 transmits the SIB including the RAN pagingarea identifier (Paging Area ID) to which its own cell (or its own eNB)belongs. The UE 100 receives the SIB.

In step S106, the UE 100 reads out the held RAN paging area identifier,and compares it with the RAN paging area identifier broadcast from thecurrent (new) cell (that is, the RAN paging area identifier received instep S105).

When the identifiers are different (step S106: No), in step S107, the UE100 transmits a notification indicating that the UE 100 has moved out ofthe RAN paging area configured in the UE 100 itself (Report it movedacross the configured paging area) to a current (new) cell. On the otherhand, when “Yes” is determined in step S106, the UE 100 does not givethe notification.

Operation Pattern 2

FIG. 8 is a diagram illustrating an operation example of the UE 100according to the operation pattern 2 of the first embodiment. In FIG. 8,processes similar to those in the operation pattern 1 illustrated inFIG. 6 are assigned the same step numbers as in FIG. 6.

In step S12, the UE 100 (the receiver 110) receives, from the servingcell, unicast signaling that causes the UE 100 to transition to theLight Connection state. The unicast signaling may be unicast RRCsignaling (the RRC reconfiguration message or the RRC release message).In the operation pattern 2, the unicast signaling includes informationindicating whether or not the RAN paging area configured in the UE 100includes only the current serving cell.

In step S13, the UE 100 (the controller 130) determines, on the basis ofthe information included in the unicast signaling, whether or not theRAN paging area configured in the UE 100 itself includes only thecurrent serving cell.

If “NO” is determined in step S13, in step S10, the UE 100 (the receiver110) receives, from the serving cell, broadcast signaling including theidentifier of the RAN paging area to which the serving cell belongs. Thebroadcast signaling may be broadcast RRC signaling (system informationblock (SIB)). The UE 100 (the controller 130) stores the received RANpaging area identifier. Then, in step S14, the UE 100 (the controller130) transitions to the Light Connection state, and holds the identifierin the broadcast signaling (that is, the RAN paging area identifierreceived in step S10) as the RAN paging area identifier configured inthe UE 100.

If “YES” is determined in step S13, in step S15, the UE 100 (thecontroller 130) transitions to the Light Connection state, and holds theidentifier (the cell identifier) of the current serving cell as the RANpaging area identifier configured in the UE 100. The UE 100 may holdinformation indicating that the RAN paging area configured in the UE 100includes only the current serving cell.

First Modified Example of First Embodiment

A first modified example of the first embodiment will be describedmainly with differences from the first embodiment.

A basic operation of the UE 100 according to the modified example of thefirst embodiment is similar to that of the first embodiment. The UE 100(the receiver 110) receives, from the eNB 200, a unicast signal thatcauses the UE 100 to transition to the Light Connection state (thespecific state). The UE 100 (the receiver 110) further receives the RANpaging area identifier broadcast by the eNB 200. The UE 100 (thecontroller 130) causes the UE 100 to transition to the Light Connectionstate in response to the reception of the unicast signal. The LightConnection state is a state in which paging area information indicatinga paging area used in an eNB-initiated paging scheme (that is, RANpaging) is configured in the UE 100. The paging area information is alsoreferred to as a paging area configuration.

In the modified example of the first embodiment, the UE 100 (thecontroller 130) holds the RAN paging area identifier as the paging areainformation configured in the UE 100 (the controller 130) when apredetermined condition is satisfied. The predetermined condition may beone of the following first to third conditions or may be a combinationof two or more conditions.

The first condition is a condition that the RAN paging area identifieris broadcast from the eNB 200. When the RAN paging area identifier isbroadcast from the eNB 200, the UE 100 recognizes that the RAN pagingarea identifier is designated as the paging area information from theeNB 200.

The second condition is a condition that the RAN paging area identifieris designated as the paging area information from the eNB 200. Suchdesignation may be performed by the unicast signal (for example, the RRCConnection Release message) that causes the transition to the LightConnection state. For example, when the RRC Connection Release messageincludes information such as “ran-pagingAreaId=TRUE,” the UE 100recognizes that the RAN paging area identifier is designated as thepaging area information from the eNB 200.

The third condition is a condition that information other than the RANpaging area identifier is not designated as the paging area informationfrom the eNB 200. The information other than the RAN paging areaidentifier is, for example, at least one of a cell list, a single cell,and the tracking area. When the information other than the RAN pagingarea identifier is not designated as the paging area information fromthe eNB 200, the UE 100 recognizes that the RAN paging area identifieris designated as the paging area information from the eNB 200.

Second Modified Example of the First Embodiment

In the first modified example of the first embodiment, the example inwhich the RAN paging area identifier is configured in the UE 100 as thepaging area information has been described, but a case in which the celllist, the single cell, or the tracking area is configured in the UE 100may be considered. For example, when the cell list or the tracking areais not designated as the paging area information from the eNB 200, theUE 100 may recognize that the single cell is designated as the pagingarea information from the eNB 200. In this case, the UE 100 may holdinformation of the single cell as the paging area information configuredin the UE 100 itself.

Second Embodiment

The description will proceed mainly with differences between the secondembodiment and the first embodiment.

FIG. 9 is a diagram illustrating an assumed scenario according to thesecond embodiment. The UE 100 in the Light Connection state moves acrossa plurality of cells. However, there may be cells (the eNB 200) havingno function of handling the Light Connection state. Examples of thefunction of handling the Light Connection state include a RAN pagingfunction, a function of recovering the UE 100 from the Light Connectionstate, and the like. Hereinafter, a cell that does not have such afunction is referred to appropriately as a “cell that does not supportLight Connection.”

In FIG. 9, among cells A to C managed by eNBs 200-1 to 200-3, the cell Bis a cell that does not support the Light Connection. The UE 100 movesin the order of the cells A, B, and C. Each eNB 200 (each cell) maybroadcast information indicating whether or not the eNB 200 supports theLight Connection. As an example, the eNB 200 (transmitter 210) transmitsthe information through the SIB. Such information may be implicitinformation. For example, the UE 100 (the controller 130) may regard acell that broadcasts the RAN paging area identifier as the cellsupporting the Light Connection. Alternatively, when a selected cell isincluded in a list of Cell IDs configured in the UE (a list of cellsincluded in the RAN paging area), the selected cell may be regarded asthe cell supporting the Light Connection, and when the selected cell isnot included, the selected cell may be regarded as the cell notsupporting the Light Connection.

In the second embodiment, the UE 100 (the controller 130) determineswhether or not the serving cell supports the function of handling theLight Connection state in the Light Connection state in which the RANpaging area is configured. The UE 100 (the controller 130) maintains theLight Connection state in the serving cell even when the serving cell isdetermined not to support the function. Therefore, the UE 100 in theLight Connection state maintains the Light Connection state even whenthe UE 100 moves from the cell A to the cell B illustrated in FIG. 9. Inother words, even though the UE 100 reselects a cell that does notsupport the Light Connection, the UE 100 continues the Light Connectionoperation without interrupting the Light Connection operation.

As a comparative example, a case in which when the UE 100 moves from thecell A to the cell B, the Light Connection operation is stopped, and anormal RRC idle mode operation is performed is considered. However, if ascenario in which the UE 100 subsequently moves from the cell B to thecell C is considered, it is desirable for the UE 100 to continue theLight Connection operation in the cell B.

In the second embodiment, the UE 100 (the controller 130) detects apredetermined event to recover from the Light Connection state. Therecovery from the Light Connection state means stopping the LightConnection operation and performing the normal RRC connected modeoperation. The predetermined event may be reception of paging by the UE100 or the occurrence of the necessity to transmit data or signaling bythe UE 100.

In a case in which the UE 100 (the controller 130) determines that theserving cell supports the Light Connection, the UE 100 transmits, to theserving cell, a first RRC message for requesting the recovery from theLight Connection state in response to the detection of the predeterminedevent. The first RRC message may be an RRC recovery (resume) requestmessage or may be an RRC reestablishment request message.

In a case in which the UE 100 (the controller 130) determines that theserving cell does not support the Light Connection, the UE 100 (thecontroller 130) transmits a second RRC message for requestingestablishment of the RRC connection with the serving cell in response tothe detection of the predetermined event. The second RRC message may bean RRC connection request message. In other words, in order to recoverfrom the Light Connection state in the cell that does not support theLight Connection, the UE 100 (the controller 130) regards that the UE100 itself is in the RRC idle mode and transmits the RRC connectionrequest message. Since the RRC connection request message is a messageused when the general UE 100 transitions from the RRC idle mode to theRRC connected mode, the message can be handled even in the cell thatdoes not support the Light Connection.

Further, when the UE 100 in the Light Connection State does not monitortraditional paging using the tracking area (that is, MME-initiatedpaging) in the cell that does not support the Light Connection, the UE100 is unlikely to be called. In this regard, the UE 100 in the LightConnection state monitors the RAN paging when the currently selectedcell supports the Light Connection, and monitors the traditionalMME-initiated paging when the currently selected cell does not supportthe Light Connection. Alternatively, the UE 100 in the Light Connectionstate may monitor both the RAN paging and the MME-initiated paging underthe cell supporting the Light Connection.

In the MME-initiated paging, the UE 100 performs the monitoring at atiming specified by the international mobile subscriber identity (IMSI)serving as UE_ID which is its identifier, for example, “SFN mod T=(T divN)*(UE_ID mod N).” On the other hand, the monitoring timing in the RANpaging may be defined at a timing different from the monitoring timingin the MME-initiated paging (at least a different parameter can beconfigured).

In the MME-initiated paging, the UE 100 detects an identifier (forexample, the IMSI or the S-TMSI) which is configured in advance orallocated by a core network as an identifier (that is, a paging record)related to paging (calling). On the other hand, in the RAN paging, theUE 100 detects an identifier (for example, “Cell ID+C-RNTI,” Resume ID,or the like) allocated (or managed) by a RAN node (for example, a basestation).

Modified Example of Second Embodiment

A modified example of the second embodiment will be described mainlywith differences from the second embodiment.

The UE 100 (the controller 130) according to the modified example of thesecond embodiment determines whether or not the current serving cell hasa function of handling the Light Connection state (that is, whether ornot the current serving cell supports the Light Connection) in the LightConnection state in which the paging area information indicating thepaging area used in a first paging scheme initiated by the eNB (that is,the RAN paging) is configured in the UE 100. When the current servingcell does not support the Light Connection, the UE 100 (the controller130) performs at least one of the following first to third processes.

The first process is a process of determining that the procedure forrequesting the recovery from the Light Connection state is unable to beperformed in the current serving cell. The procedure may be the RRCResume procedure. The first process may be a process of determining thatthe recovery request message (the first RRC message described above) isnot permitted to be transmitted to the current serving cell. In otherwords, the UE 100 can transmit the recovery request message to thecurrent serving cell only when the current serving cell supports theLight Connection. The recovery request message may include RAN pagingarea update information related to RAN paging area update (PAU). The RANpaging area update information may be included in the recovery requestmessage. The RAN paging area update information may be included in amessage (for example, an RRC Resume Complete message) transmitted afterthe transmission of the recovery request message.

The second process is a process of transitioning to the RRC idle mode.In the UE 100, the AS layer may give a notification indicating “RRCConnection failure” to the NAS (upper layer). As a result, the UE 100transitions to the RRC idle mode. The AS layer may give a notificationindicating “RRC Connection failure” to the NAS layer immediately after acell that does not support the Light Connection is reselected by thecell reselection operation. The AS layer may give a notificationindicating “RRC Connection failure” to NAS layer when Mobile Terminated(MT) and/or Mobile Originated (MO) occurs after the cell that does notsupport the Light Connection is reselected by the cell reselectionoperation. The MT includes paging reception. The MO includes theoccurrence of the necessity of uplink data or signaling. The UE 100 maydetermine whether or not the current serving cell supports the LightConnection when the MT and/or the MO occur. The UE 100 may check the SIBof the current serving cell when the MT and/or the MO occurs and give anotification indicating “RRC Connection failure” to the NAS layer if anotification indicating Light Connection support is not given.

The third process is a process of monitoring paging using a secondpaging scheme initiated by the core network (that is, the MME-initiatedpaging). Such paging may be paging for the RRC idle mode. Specifically,the UE 100 monitors paging in accordance with paging occasions (PF/PO)of DRX for the idle mode. Such a paging occasion will be described in afifth embodiment. Also, in the third process, the UE 100 receives thepaging message, checks the international mobile subscriber identity(IMSI) and/or the SAE temporary mobile subscriber identity (S-TMSI) inthe received paging message, and determines whether or not the pagingmessage is addressed to the UE 100 itself. In the third process, the UE100 may not check the identifier (for example, the Resume ID) for theRAN paging for the received paging message. In the third process, the UE100 may maintain the Light Connection state (that is, not transition tothe RRC idle mode and not return to the normal RRC connected mode). TheUE 100 may leave the Light Connection state and return to the normal RRCconnected mode when the MT or the MO occurs. Alternatively, when the MTor the MO occurs, the UE 100 may transition to the RRC idle mode, forexample, by giving the notification indicating “RRC Connection Failure”from the AS layer to the NAS layer.

The UE 100 may determine that the message used for the Light Connectionstate is unable to be transmitted to the current serving cell as thecurrent serving cell does not support the Light Connection. Such amessage may be, for example, a notification message indicating that nodata communication occurs (or is unlikely to occur) in the UE 100.

Third Embodiment

A third embodiment will be described mainly with differences from thefirst and second embodiments. The third embodiment is an embodimentrelated to the cell reselection operation of the UE 100 in the LightConnection state.

The UE 100 according to the third embodiment includes a controller 130that performs the cell reselection operation in the Light Connectionstate. The controller 130 preferentially selects, as the serving cell ofthe UE 100, the cell that supports the recovery from the LightConnection state in the cell reselection operation.

A general cell reselection operation is an operation of selecting anappropriate cell in accordance with ranking based on a priority of afrequency to which the cell belongs and a radio quality of the cell.

In the cell reselection operation according to the third embodiment, theUE 100 (the controller 130) may configure the cell supporting the LightConnected as the highest priority. The UE 100 (the controller 130) mayconfigure the cell that does not support the Light Connected as thelowest priority. Here, Highest/Lowest means priorities (for example, “8”and “−1”) higher/lower than a priority (CellReselectionPriority: 0 to 7)broadcast from the eNB 200 or a value obtained by adding the priorityand the sub priority (CellReselectionSubPriority: 0.2, 0.4, 0.6, and0.8).

In the cell reselection operation according to the third embodiment, theUE 100 (the controller 130) may give a priority to the cell supportingthe Light Connected by introducing an offset into the ranking. Forexample, a positive offset is added to the cell supporting the LightConnected, and/or a negative offset is added to the cell that does notsupport the Light Connected. The offset value may be a value which isdefined in advance or may be a value which is configured from the eNB200. When the offset value is configured from the eNB 200, the eNB 200may broadcast the offset value. When the offset value is configured fromthe eNB 200, the eNB 200 may configure the offset value throughdedicated signaling for each UE.

In the third embodiment, the eNB 200 may configure, in the UE 100,whether or not priority control of the cell supporting the recovery fromthe Light Connection state is performed. The configuration may beperformed when transition to the Light Connection state is performed.The configuration may be included in the RRC Connection Reconfigurationor the RRC Connection Release.

Each eNB 200 (each cell) may broadcast information indicating whether ornot the Light Connection state (specifically, the recovery from theLight Connection state) is supported. As an example, the eNB 200transmits the information through the SIB. Such information may beimplicit information. For example, the UE 100 may regard the cell thattransmits the identifier of the RAN paging area as the cell supportingthe Light Connection state.

In the third embodiment, the UE 100 may support the recovery from theLight Connection state and transition to the RRC idle mode as the cellthat does not satisfy a predetermined radio quality standard (forexample, S criterion) is not detected. As an example, the UE 100 maytransition to the RRC idle mode, for example, when the cell satisfyingthe S criterion is only a legacy cell (that is, a cell not supportingthe Light Connection state).

Modified Example of Third Embodiment

A modified example of the third embodiment will be described mainly withdifferences from the third embodiment.

The UE 100 (the controller 130) according to the modified example of thethird embodiment determines whether or not an adjacent cell which is amovement destination of the UE 100 has a function of handling the LightConnection state (whether or not the adjacent cell supports the LightConnection) in the Light Connection state in which the paging areainformation indicating the paging area used in the first paging schemeinitiated by the eNB (the RAN paging) is configured in the UE 100. TheUE 100 (transmitter 120) transmits the recovery request message to thecurrent serving cell before the cell reselection of changing the servingcell of the UE 100 from the current serving cell to the adjacent cell asthe adjacent cell does not support the Light Connection. The recoveryrequest message is a message (RRC Connection Resume message) forrequesting the recovery from the Light Connection state.

In the modified example of the third embodiment, the UE 100 (thereceiver 110) may receive information indicating whether or not theadjacent cell supports the Light Connection from the current servingcell. The UE 100 (the controller 130) may determine whether or not theadjacent cell supports the Light Connection on the basis of theinformation received from the current serving cell.

FIG. 10 is a diagram illustrating an operation example according to themodified example of the third embodiment.

In step S301, the UE 100 in the Light Connection state determineswhether or not it is necessary to perform the cell reselection to theadjacent cell on the basis of the determination result or the like forthe adjacent cell. The determination is performed at a predeterminedtiming before the cell reselection is actually performed. Thepredetermined timing may be a timing immediately before the cellreselection or may be a timing with a certain grace period. Thedetermination result may be a determination result of a reception level(reference signal received power (RSRP)). The determination result maybe a determination result of reference signal received power (RSRQ)and/or a reference signal-signal to noise and interference ratio(RS-SINR). The UE 100 may recognize that it is immediately before thecell reselection in accordance with a threshold value (for example, anRSRP threshold value, an RSRQ threshold value, and/or an RS-SINRthreshold value) configured from the eNB 200. The threshold value may beconfigured through unicast signaling (for example, the RRC ConnectionRelease message) indicating the Light Connection or may be broadcastthrough the SIB.

When the UE 100 determines that it is necessary to perform the cellreselection to the adjacent cells (step S301: YES), in step S302, the UE100 determines whether or not the adjacent cell supports the LightConnection. The UE 100 receives, from the current serving cell, a listindicating whether or not the adjacent cell supports the LightConnection. The UE 100 may perform the determination on the basis of thelist. The list may be a list of identifiers of adjacent cells thatsupport the Light Connection. The list may be a list of identifiers ofadjacent cells that do not support the Light Connection. Alternatively,the UE 100 may perform the determination on the basis of the information(for example, the SIB) broadcast from the adjacent cell. The informationbroadcast from the adjacent cell may be information indicating whetheror not the Light Connection is supported. The process of S302 may beperformed before S301.

When the UE 100 determines that the adjacent cell supports the LightConnection (step S302: YES), in step S303, the UE 100 performs the cellreselection to the adjacent cell.

When the UE 100 determines that the adjacent cell does not support theLight Connection (step S302: NO), in step S304, the UE 100 transmits therecovery request message to the current serving cell in order totransition to the normal RRC connected mode.

The recovery request message may include an identifier (for example, theResume ID) which is allocated from the eNB 200 to the UE 100 when the UE100 transitions to the Light Connection state. The identifier may beconfigured in the UE 100 through unicast signaling (for example, the RRCConnection Release message) indicating the Light Connection.

The recovery request message may also include information indicatingthat the adjacent cell does not support the Light Connection. Theinformation may be included in the recovery request message as “ResumeCause” indicating the reason for the recovery request.

The recovery request message may include identification information foridentifying an adjacent cell. The identification information is PhysicalCell Identity (PCI), E-UTRAN Cell Global Identifier (ECGI), and/or thelike. The recovery request message may include the determination resultfor the adjacent cell. The determination result may be the RSRP. Thedetermination result may be a reference signal received quality (RSRQ).The identification information and/or the determination result of theadjacent cell may be included in the recovery request message only whenan instruction is given from the eNB 200 (for example, the currentserving cell) through unicast signaling or the SIB. The UE 100 may notinclude the identification information and/or the determination resultof the adjacent cell in the recovery request message when there is noinstruction from the eNB 200. For example, when the eNB 200 is scheduledto trigger a measurement report of the UE 100 immediately after the UE100 returns to the normal RRC connected mode, the eNB 200 does not givethe instruction.

As a result of step S304, the UE 100 transitions to the normal RRCconnected mode. Thereafter, the current serving cell (the eNB 200) maycause the UE 100 to transition to the RRC idle mode by transmitting theRRC Connection Release message to the UE 100. The eNB 200 may transmitan RRC connection reject message to the UE 100 and cause the UE 100 totransition from the Light Connection state to the RRC idle mode withoutcausing the UE 100 to transition to the normal RRC connected mode. Aftercausing the UE 100 to transition to the RRC connected mode, the eNB 200may hand the UE 100 over to the adjacent cell on the basis of theidentification information and/or the determination result of theadjacent cell included in the recovery request message. The eNB 200 mayconsider a past data communication history or the like of the UE 100 indetermining whether or not the handover of the UE 100 is performed. TheeNB 200 may delete context information (UE context) of the UE 100 thathas transmitted the recovery request message or may transfer it to theadjacent cell (the adjacent eNB).

Fourth Embodiment

A fourth embodiment will be described mainly with differences from thefirst to third embodiments. The fourth embodiment is an embodimentrelated to an operation of performing the RAN paging through the eNB200.

The eNB 200 according to the fourth embodiment includes a controller 230that performs the RAN paging on the UE 100 in the Light Connection stateand determines whether or not the RAN paging is successfully performedand a transmitter (a backhaul communication unit 240) that transmits afailure notification indicating a failure of the RAN paging to the MME300C in accordance with the failure of the RAN paging. The RAN paging isan operation in which the RAN performs the paging of the UE 100 in unitsof RAN paging areas. The failure notification may be a message forcausing the MME 300C to execute the paging based on the tracking area inwhich the UE 100 exists. Therefore, even when the RAN paging fails, theMME 300C can execute the normal paging.

The eNB 200 according to the fourth embodiment may include a receiver (abackhaul communication unit 240) that receives, from another the eNB200, information indicating whether or not another eNB 200 in the RANpaging has performed paging successfully. The controller 230 determinesthat the RAN paging has failed as both its own the eNB 200 and anotherthe eNB 200 have failed in paging.

FIG. 11 is a diagram illustrating an operation example according to thefourth embodiment. In FIG. 11, an anchor eNB 200-1 and an eNB 200-2belong to the same RAN paging area. The anchor eNB 200-1 and the eNB200-2 may be connected via the X2 interface. In the initial state, theUE 100 is in the RRC connected mode (S2001 and S2002). In FIG. 11, anoperation indicated by a broken line is not essential.

The operations of steps S2003 and S2004 are similar to those in thefirst embodiment.

In step S2005, the anchor eNB 200-1 receives data (DL data) addressed tothe UE 100 from an S-GW 300U via an S1 connection for the UE 100. Theanchor eNB 200-1 determines to start the paging of the UE 100 inresponse with the reception of the data.

In step S2006, the anchor eNB 200-1 transmits, to the eNB 200-2, apaging request for requesting execution of the paging (RAN paging) ofthe UE 100. The paging request may include information specifying apaging timing (see the fifth embodiment).

In step S2007, the anchor eNB 200-1 causes a timer to start when thepaging is determined to start or when the paging request is transmitted.The anchor eNB 200-1 may cause the timer to be stopped upon receiving apaging response from the UE 100 or upon receiving a paging successnotification from the eNB 200-2.

In step S2008, the anchor eNB 200-1 and the eNB 200-2 transmit a pagingmessage (Ran paging) addressed to the UE 100 in the RAN paging areaconfigured in the UE 100. Here, the description will proceed assumingthat the UE 100 has failed to receive the paging message (Ran paging).

In step S2009, the eNB 200-2 transmits, to the anchor eNB 200-1, afailure notification (Paging Failure) indicating that the UE 100 hasfailed in paging (RAN paging).

In step S2010, the anchor eNB 200-1 determines whether or not the timerhas expired and/or whether or not the failure notification (PagingFailure) has been received. Here, the description will proceed assumingthat the timer has expired and/or the failure notification (PagingFailure) has been received.

In step S2011, the anchor eNB 200-1 transmits the failure notification(RAN Paging Failure) indicating the failure of the RAN paging to the MME300C on the S1 interface. The failure notification (RAN Paging Failure)includes an identifier (for example, an eNB UE SlAP ID) used for the MME300C to identify the UE 100. The failure notification (RAN PagingFailure) may include an MME UE SlAP ID, a Cause (for example, RAN PagingFailed), or the like. Instead of the failure notification (RAN PagingFailure), the paging request for requesting execution of paging may beused.

In step S2012, in response to the reception of the failure notification(RAN Paging Failure) from the anchor eNB 200-1, the MME 300C transmitsthe paging message (PAGING) to each eNB 200 belonging to the trackingarea in which the UE 100 exists. Each eNB 200 belonging to the trackingarea in which the UE 100 exists transmits the paging message in its owncell. The MME 300C may transmit the paging message only to some eNBs 200belonging to the tracking area instead of transmitting the pagingmessage to all the eNBs 200 belonging to the tracking area in which theUE 100 exists.

In step S2013, in response to reception of the paging message (PAGING),the UE 100 transmits the message (RRC Connection Boot request) forrequesting the recovery from the Light Connection state to the eNB 200(for example, the anchor eNB 200-1).

First Modified Example of Fourth Embodiment

A first modified example of the fourth embodiment will be describedmainly with differences from the fourth embodiment.

FIG. 12 is a diagram illustrating an operation example according to thefirst modified example of the fourth embodiment. The eNBs 200-1 to 200-nillustrated in FIG. 12 are eNBs 200 belonging to the same tracking area.

As illustrated in FIG. 12, in step S201, the eNB 200-1 causes the UE 100to transition to the Light Connection state.

In step S202, the eNB 200-1 transmits, to the MME 300C, a transitionnotification (Light Connection Indication) related to the UE 100 totransition to the Light Connection state. The transition notificationincludes an identifier (for example, an MME UE SlAP ID) for identifyingthe UE 100 (a specific UE 100) to transition to the Light Connectionstate.

In step S203, in response to the reception of the transitionnotification, the MME 300C transmits a monitoring request (Notify DLdata Request) to the S-GW 300U. The monitoring request is a message forrequesting to monitor whether or not there is DL data addressed to aspecific UE 100. The message includes an identifier (for example, GTPTEID) for identifying the specific UE 100. The S-GW 300U startsmonitoring DL data addressed to the specific UE 100 in response to thereception of the monitoring request. When the monitoring is started, theS-GW 300U may transmit an acknowledgment (ACK) to the monitoring requestto the MME 300C.

In step S204, the S-GW 300U receives the DL data addressed to thespecific UE 100 from the P-GW. The S-GW 300U may transfer the DL dataaddressed to the specific UE 100 to the eNB 200-1 using the S1-Uconnection (step S205). Alternatively, the S-GW 300U may temporarilystop the transfer of the DL data addressed to the specific UE 100. TheMME 300C may transmit a monitoring request for requesting such a stop tothe S-GW 300U using the same message as the monitoring request or amessage different from the monitoring request.

When the DL data addressed to the specific UE 100 is received (stepS206: YES), in step S207, the S-GW 300U transmits a notification (NotifyDL data) indicating that there is DL data addressed to the specific UE100 to the MME 300C.

In step S208, the MME 300C determines whether or not it is necessary totransmit DL data or NAS signaling addressed to a specific UE 100. TheMME 300C determines, on the basis of the notification from the S-GW300U, whether or not it is necessary to transmit the DL data addressedto the specific UE 100.

If “YES” is determined in step S208, the MME 300C determines aregistered tracking area of the specific UE 100 on the basis of the UEidentifier included in the transition notification, and transmits an S1paging message to the eNB 200 belonging to the registered tracking area.Thereafter, the paging procedure and the RRC connection resume procedureare performed.

Second Modified Example of Fourth Embodiment

A second modified example of the fourth embodiment will be describedmainly with difference from the fourth embodiment and the first modifiedexample thereof.

In the fourth embodiment, the example of performing the paging (MMEpaging) using the second paging scheme initiated by the core network(MME) in response to the failure of the paging (RAN paging) using thefirst paging scheme initiated by the eNB has been described. Themodified example of the fourth embodiment is a modified example in whichthe MME paging is performed in parallel with the RAN paging.

The eNB 200 (controller 230) according to the second modified example ofthe fourth embodiment performs the first paging (the RAN paging) for theUE 100 using the first paging scheme initiated by the eNB. The eNB 200according to the modified example of the fourth embodiment may be ananchor eNB. When the eNB 200 (controller 230) determines that the RANpaging is unlikely to reach the UE 100, the eNB 200 (controller 230)give a notification indicating predetermined information causing the UE100 to start the MME paging to the core network (MME). The UE 100(controller 230) gives a notification indicating predeterminedinformation to the core network (MME) at a timing before the RAN pagingis executed or while the RAN paging is being. The predeterminedinformation may be an MME paging execution request. The predeterminedinformation may be a notification similar to the first modified exampleof the fourth embodiment.

In the second modified example of the fourth embodiment, the eNB 200(controller 230) may determine that the RAN paging is unlikely to reachthe UE 100 when there is an area which is not covered by the paging area(that is, the RAN paging area) used in the RAN paging within apredetermined geographical area. The predetermined geographical area maybe an area of a geographical range corresponding to the RAN paging area.The predetermined geographical area may be a tracking area. When thereis a cell that does not support the RAN paging among the cells includedin the predetermined geographical area, the geographical range of thecell is not covered by the RAN paging area. In other words, a coveragehole of the RAN paging occurs within the predetermined geographicalarea. In such a case, the eNB 200 (controller 230) determines that theRAN paging is unlikely to reach the UE 100, and performs control suchthat the MME paging is performed in parallel with the RAN paging.

When the Light Connection is configured in the UE 100, the eNB 200 maygive a notification indicating that configuration to the MME 300C. Whenthe eNB 200 determines that the UE 100 supports the Light Connectionbefore the Light Connection is configured in the UE 100, the eNB 200 maygive a notification indicating that the UE 100 supports the LightConnection to the MME 300C. The eNB 200 may give a notification to theMME 300C on the basis of the configuration from the OAM.

After the notification is received, the MME 300C may transmit the S1paging message in response to the notification from the S-GW 300U (seestep S207 in FIG. 12). The MME 300C may transmit the S1 paging messageperiodically.

When the DL data arrives (see step S204 in FIG. 12), the S-GW 300U maytransfer the DL data to the anchor eNB for the RAN paging and hold (thecopy of) the DL data for the MME paging.

The eNB 200 that has received the S1 paging message from the MME 300Cmay merge the received S1 paging message (the MME paging message) andthe RAN paging message to generate a paging message (the RRC message) tobe transmitted to the UE 100. For example, the paging message (the RRCmessage) including both the IMSI in the S1 paging message and the ResumeID in the RAN paging message may be generated.

The UE 100 receives only the MME paging message, receives only the RANpaging message, or receives both messages. Alternatively, the UE 100 mayreceive a paging message in which the MME paging message and the RANpaging message are merged. The UE 100 may check the UE identificationinformation (for example, the TMGI, the S-TMSI, or the Resume ID) in thereceived paging message and determine a type of paging message inaccordance with the type of UE identification information.

For example, if the received paging message includes the Resume ID (andthe S-TMSI) allocated to the UE 100 itself, the UE 100 may determinethat the RAN paging message has been received. The UE 100 may determinethat the MME paging message has been received if the received pagingmessage does not include the Resume ID (and the S-TMSI) allocated to theUE 100 itself. The UE 100 may start the RRC Connection Resume procedurein response to the reception of the paging message.

The UE 100 may transmit a response corresponding to the type of receivedpaging message to the network. For example, when the UE 100 determinesthat the MME paging message has been received, the UE 100 may transmit apaging response (NAS signaling) to the MME 300C. When the UE 100determines that the RAN paging message has been received, the UE 100 maytransmit the RRC Connection Resume message to the eNB 200. The UE 100may include information (Cause=MT access) indicating that the paging hasbeen received in the RRC Connection Resume message. The UE 100 mayinclude the Resume ID allocated to the UE 100 itself in the RRCConnection Resume message.

When UE 100 determines that the paging messages of both the RAN pagingand the MME paging (or the merged paging messages) have been received,the UE 100 may give a priority to the response to the MME paging or maygive a priority to the response to the RAN paging. When UE 100determines that the paging messages of both the RAN paging and the MMEpaging (or the merged paging messages) have been received, the UE 100may transmit two responses corresponding to both of the paging messages.

When the paging response is received from the UE 100, the MME 300C maygive a notification indicating that the response has been received tothe anchor eNB. When the eNB 200 receives the response to the RAN pagingfrom the UE 100, the eNB 200 may give a notification indicating that theresponse has been received to the MME 300C.

Third Modified Example of Fourth Embodiment

The MME 300C may give a notification to the anchor eNB when a trackingarea update message (NAS signaling) is received from the UE 100. The MME300C may transmit the notification only when the RAN paging area isconfigured similarly to the tracking area. The MME 300C may transmit thenotification only when the notification indicating that the UE 100 isconfigured to the Light Connection is received from the eNB 200. The MME300C may include the identifier of the eNB to which the UE 100 iscurrently connected in the notification. The anchor eNB may transfer theUE context to the eNB to which the UE 100 is currently connected on thebasis of the notification. Alternatively, the notification may be a UEContext Release. The eNB 200 that has received the UE Context Releasereleases the UE context.

Fifth Embodiment

A fifth embodiment will be described mainly with differences from thefirst to fourth embodiments. The fifth embodiment is an embodimentregarding the DRX operation of the UE 100 in the Light Connection state.

A general idle mode DRX operation will be described. In order to reducepower consumption, discontinuous reception (DRX) may be configured inthe UE 100. In the DRX operation, the UE 100 in the RRC idle modemonitors the paging message in a paging reception opportunity (pagingoccasion) that occurs at a predetermined time interval (DRX cycle). Inthe DRX operation, UE 100 intermittently monitors the PDCCH to receivepaging. The UE 100 decodes the PDCCH using an identifier for paging(paging radio network temporary identifier (P-RNTI)), and acquiresallocation information of a paging channel. The UE 100 acquires thepaging message on the basis of the allocation information. A PDCCHmonitor timing in the UE 100 is decided on the basis of the identifier(international mobile subscriber identity (IMSI)) of the UE 100. ThePDCCH monitor timing (PDCCH monitor sub frame) in the DRX operation isreferred to as a paging occasion (PO). The PO corresponds to a pagingreception opportunity.

The UE 100 and the eNB 200 calculate the paging occasions (PO) and apaging frame (PF) which is a radio frame that may include the pagingoccasion as follows.

A system frame number (SFN) of the PF can be obtained from the followingFormula (1).SFN mod T=(T div N)*(UE_ID mod N)  (1)

T is the DRX cycle of the UE 100 for monitoring paging. T is indicatedby the number of radio frames. Also, T is a smaller one out of a defaultDRX value broadcast from the eNB 200 through the system informationblock (SIB) and a UE-specific DRX value configured in the UE 100 throughthe NAS message. When the UE-specific DRX value is not configured, UE100 applies the default DRX value. N is a minimum value out of T and nB.nB is a value selected from 4T, 2T, T, T/2, T/4, T/8, T/16, and T/32.UE_ID is a value obtained by “IMSI mod 1024.”

Among the PFs obtained as described above, an index i_s is obtained bythe following Formula (2), and a sub frame number of the POcorresponding to the index i_s is obtained.i_s=floor(UE_ID/N)mod Ns  (2)

Here, Ns is a maximum value out of 1 and nB/T.

An operation according to the fifth embodiment will be described. FIG.13 is a diagram illustrating an operation example according to the fifthembodiment.

The UE 100 according to the operation pattern 1 of the fifth embodimentincludes a receiver 110 that a transition instruction for instructingtransition to the Light Connection state from the serving cell and acontroller 130 that transitions to the Light Connection state in theserving cell, and performs the DRX operation of the RRC connected mode.As illustrated in FIG. 13(a), the UE 100 continues the DRX operation ofthe RRC connected mode while the UE 100 exists in the serving cell atthe time of transition to the Light Connection state. As illustrated inFIG. 13(b), the controller 130 of the UE 100 stops the DRX operation ofthe RRC connected mode as the UE 100 moves from the serving cell toanother cell in the RAN paging area. The controller 130 of the UE 100stops the DRX operation of the RRC connected mode and starts anoperation based on the DRX operation of the RRC idle mode. The operationbased on the DRX operation of the RRC idle mode is an operation ofdeciding the PF and the PO by the formula of the paging frame (PF) andthe paging occasion (PO) in the DRX operation of the RRC idle mode or aformula using it. As illustrated in FIG. 13(c), the controller 130 ofthe UE 100 gives a notification when the UE 100 moves to a different RANpaging area.

Even when the UE 100 according to the operation pattern 2 of the fifthembodiment moves from the serving cell to another cell at the time oftransition to the Light Connection state, if the other cell belongs tothe same the RAN paging area, the UE 100 continues the DRX operation ofthe RRC connected mode. In this case, as illustrated in FIGS. 13(a) and(b), the UE 100 can continue the DRX operation of the RRC connected modein the same RAN paging area. That is, even when the UE 100 moves toanother cell, the UE 100 performs the reception operation in accordancewith the connected mode DRX.

Here, such an operation may be performed in units of eNBs 200. That is,in the operation patterns 1 and 2 of the fifth embodiment, the “servingcell” may be read as a “serving eNB” or an “anchor eNB,” and “anothercell” may be read as an “another eNB.”

In the operation patterns 1 and 2 of the fifth embodiment, the UE 100other than the anchor eNB need not necessarily hold the contextinformation of the UE 100. Therefore, it is desirable for other eNBs inthe same the RAN paging area to acquire information for deciding thepaging timing from the anchor eNB. The other eNBs perform the RAN pagingfor the UE 100 in the Light Connection state. The other eNBs acquireinformation for deciding a timing of transmitting the paging message forthe RAN paging to the UE 100 from the anchor eNB. The information fordeciding the timing includes at least one of identification informationof the UE 100 (for example, the IMSI, the S-TMSI, the Resume ID, or thelike) and the DRX configuration of the RRC connected mode. The anchoreNB may include such information in the paging request and transmit itto other eNBs.

The identification information for deciding the paging timing may be anE-UTRAN cell global identifier (ECGI) and a cell-radio network temporaryidentifier (C-RNTI). The anchor eNB 200-1 may allocate theidentification information to the UE 100 when causing the UE 100 totransition to the Light Connection state.

First Modified Example of Fifth Embodiment

In a case in which the timing at which the RAN paging is transmitted isspecified by the IMSI, a notification indicating the IMSI may be givenfrom the UE 100 since the eNB 200 does not know the IMSI of the UE 100for performing the RAN paging. The UE 100 may notify the base station ofits IMSI as capability information indicating that the UE 100 supportsthe Light Connection function. When the eNB 200 is notified of the IMSIin the capability information of the UE 100, the eNB 200 may determinethat the UE 100 supports the Light Connection function and use the IMSIfor specifying the RAN paging timing.

Second Modified Example of Fifth Embodiment

A second modified example of the fifth embodiment will be describedmainly with difference from the fifth embodiment and the first modifiedexample thereof.

The eNB 200 (controller 230) according to the second modified example ofthe fifth embodiment performs paging (the RAN paging) for the UE 100using an eNB-initiated paging scheme. In the eNB-initiated pagingscheme, the eNB 200 (the controller 230) determines a paging occasion(PF/PO) indicating a candidate timing for transmitting the pagingmessage to the UE 100 using the identification information foridentifying the UE 100. The eNB 200 (the controller 230) acquires theidentification information from the UE 100 or the core network (forexample, the MME 300C). The eNB 200 (the anchor eNB) may store theidentification information, the IMSI, the Resume ID, the C-RNTI, and theUE Context in association with one another.

The eNB 200 (the anchor eNB) may give a notification indicating theidentification information to the other eNBs 200 in the paging area (theRAN paging area) used in the eNB-initiated paging scheme. The eNB 200may give a notification indicating the identification informationthrough an X2 Paging message (see step S2006 in FIG. 11).

The identification information used for deciding the paging occasion(PF/PO) may be the IMSI or may be other identification information (forexample, “UE Identity Index Value” in the S1 paging message). Theidentification information may be a UE ID calculated from the IMSI usinga predetermined calculation formula (for example, IMSI mod 1024).

—Case of Acquiring Identification Information from MME 300C

The MME 300C may notify the eNB 200 of the identification informationthrough a UE CONTEXT MODIFICATION REQUEST message which is a sort of S1message. The UE CONTEXT MODIFICATION REQUEST message is a message forchanging a part of the already established UE context (that is, the UEcontext existing in the eNB 200). The MME 300C may notify the eNB 200 ofthe identification information through other S1 messages (for example,INITIAL CONTEXT SETUP REQUEST and UE RADIO CAPABILITY MATCH RESPONSE).The INITIAL CONTEXT SETUP REQUEST message is a message for requesting toestablish the UE context (that is, to generate the UE context in the eNB200). The UE RADIO CAPABILITY MATCH REQUEST message is a request messagenotified from the MME 300C to the eNB 200 in order to check whether ornot radio capability (capability) information of the UE 100 matchesbetween the MME 300C and the eNB 200.

The MME 300C may notify the eNB 200 of the identification informationonly when the UE 100 supports the Light Connection.

The MME 300C may notify the eNB 200 of the identification informationonly when the identification information is inquired from the eNB 200 (anew message). For example, the eNB 200 made an inquiry to the MME 300Cwhen the RAN paging is necessary and/or when the UE 100 transitions tothe Light Connection.

—Case of Acquiring Identification Information from UE 100

The UE 100 may notify the eNB 200 of the identification informationthrough an RRC Connection Release Complete message which is a responsemessage to the RRC Connection Release message from the eNB 200. The UE100 may include the identification information in the RRC ConnectionRelease Complete message only when the transition to the LightConnection is instructed in the RRC Connection Release message.

The UE 100 may use a procedure other than the RRC Connection ReleaseProcedure. For example, the UE 100 may use the UE Information procedurewhich is a sort of RRC procedure. The UE 100 may include theidentification information in a response (UE Information message) to aninquiry from the eNB 200. The UE 100 may use the UE Capability Transferprocedure which is a sort of RRC procedure. The UE 100 may transmit, tothe eNB 200, a UE Capability message including the identificationinformation only when the UE 100 supports the Light Connection. The UE100 may use a UE Assistance Information procedure which is a sort of RRCprocedure. The UE 100 may include the identification information in amessage used for the Light Connection state. Such a message may be, forexample, a notification message indicating that no data communicationhas occurred (or is unlikely to occur) in the UE 100.

—Case of Decrypting NAS Message

The eNB 200 decrypts the NAS message (for example, ATTACH REQUEST orATTACH ACCEPT) which is transmitted and received between the UE 100 andthe MME 300C and reads the IMSI in the NAS message. The eNB 200 mayconvert the PF/PO into numerical information which can be calculated andthen store it without storing the read IMSI without change.

Other Embodiments

In the above-described embodiment, the Public Land Mobile Network (PLMN)has not been described. The eNB 200 may configure one or more PLMNidentifiers (for example, a list of PLMN identifiers) together with theRAN paging area identifier or the cell identifier as the RAN paging areaconfigured in the UE 100. The UE 100 may recognize a cell whichbroadcasts the RAN paging area identifier or the cell identifierconfigured in the UE 100 and broadcasts the PLMN identifier configuredin itself as a cell in the RAN paging area.

The present disclosure is not limited to the case in which eachembodiment described above is implemented independently, but two or moreembodiments may be implemented in combination. For example, someoperations according to one embodiment may be added to anotherembodiment. Alternatively, some operations according to one embodimentmay be replaced with some operations of another embodiment.

In the embodiments described above, the LTE system has been described asa mobile communication system. However, the embodiments are not limitedto the LTE system. The embodiments may be applied to a system other thanthe LTE system. For example, the embodiments may be applied to the 5thgeneration communication system (5G system). In the 5G system, anInactive state (Inactive mode) is considered as a new RRC state, and theLight Connection state in the embodiments may be read as the Inactivestate. Also, an entity that performs core network paging in the 5Gsystem may be an entity other than the MME. When the embodiments areapplied to the 5G system, the RAN paging may be read as a RANnotification, and the RAN paging area may be read as a RAN notificationarea.

(Additional Note)

(1. Introduction)

In this additional note, the issues related to/out of the emaildiscussion scope are discussed.

2. Discussion

(2.1. S-TMSI Reception in RAN-Initiated Paging Message)

Discussion point 1, it was agreed a working assumption that the “ResumeID will be used in the RAN initiated paging message. UE needs to checkboth the S-TMSI and Resume ID in the paging message. FFS: UE actions onreception of its S-TMSI”. Please provide your view”.

As stated in the current specification, the paging message is used forthe UE to be informed of the paging information, SI change notification,ETWS/CMAS notifications, EAB parameter modifications and E-UTRANinter-frequency redistribution triggering. The Uu paging messages forpaging information and ETWS/CMAS notifications are normally triggered byS1 PAGING and S1 WRITE-REPLACE WARNING REQUEST respectively, while onefor the other purposes is initiated by the eNB.

From the eNB's perspective, S-TMSI is only provided within RRCConnection Request, RRC Connection Setup Complete or S1 PAGING. Asinformed by SA3, S-TSMI is frequently updated for security reason. So,the eNB may not have the knowledge of current S-TSMI for a specific UE.

Therefore, if the RAN-initiated paging message to the UE in LightConnection contains S-TMSI, the paging message is actually triggered bythe MME, i.e., S1 PAGING. It could be considered as some sort ofabnormal condition, e.g., RAN-initiated paging is unreachable for somereason and the (anchor) eNB asks the MME to initiate the legacyCN-controlled paging.

So, before to discuss on “FFS: UE actions on reception of its S-TMSI”,it needs to be clarified whether and what the case that the UE receivesits S-TMSI within RAN-initiated paging message is.

Proposal 1: RAN2 should clarify whether and what the case that the UEreceives its S-TMSI within RAN-initiated paging message is, if “FFS: UEactions on reception of its S-TMSI” needs to be identified.

FIG. 14 illustrates existing messages containing S-TMSI.

(2.2. RAN Paging Area ID)

This is related to Discussion point 4 that “Should a new RAN-configuredpaging area identifier (ID) be considered as another option to definethe RAN-configured paging area?”.

The configured RAN paging area will be one of the following options.

-   -   a list of cells    -   single cell    -   the same as CN Tracking Area    -   FFS: paging area which can be indicated by ID

These options, including the “RAN paging area ID”, could be consideredas useful for different scenarios. For example, the network may use the“list of cells” if it needs flexible configurations for each UE, whilethe network may also use the “RAN paging area ID” when it wants tominimize the signalling overhead. So, it's preferable to introduce theone more option for RAN paging area configuration.

Proposal 2: RAN2 should introduce the broadcasted RAN paging area ID.

If Proposal 2 is agreeable, the question is whether multiple RAN pagingarea IDs is useful or not. For example, if the cell may belong to twoRAN paging areas, e.g., a larger area and another smaller area, then theserving cell may select which RAN paging area is suitable for each UE,e.g., a high mobility UE may be configured with the larger area to avoidfrequent notifications; or another stationary UE may be configured withthe smaller area to reduce signalling overhead due to RAN paging. Asanother example, if it's assumed that only a single RAN paging area IDcan be broadcasted per cell, it should still be possible to configurethe UE with multiple RAN paging area IDs, so that effectively the UE'sRAN paging area is the combination of all the individual areas. In bothcases, UEs may be configured with different sizes of RAN paging area ifneeded. So, it's worth discussing whether or not to allow multiple RANpaging area IDs to be broadcasted and/or configured.

Proposal 3: RAN2 should discuss whether to allow multiple RAN pagingarea IDs (to be broadcasted and/or configured).

If it's decided that a single RAN paging area ID is only broadcasted andconfigured, i.e., Proposal 3 is not agreeable, it's not necessary toexplicitly configure the UE with RAN paging area ID when it enters LightConnection, since it's obvious the RAN paging area ID to be configuredis the same as the one broadcasted by the anchor eNB that sends the UEto Light Connection. Otherwise, some ping-pong may be concerned, e.g.,if different RAN paging area ID from one broadcasted is configured tothe UE then it should return to RRC Connected immediately to send thenotification. So, RAN-PagingArealnfo-r14 in the running CR of TS36.331should contain an indication, e.g., “ran-pagingAreald” defined asENUMERATED {true}, rather than the ID itself.

Proposal 4: If a cell can only broadcast a single RAN paging area ID,the UE will implicitly use RAN paging area ID broadcasted by the “anchoreNB” that triggers the UE to go to Light Connection, without theexplicit configuration of the RAN paging area ID.

FIG. 15 illustrates multiple RAN paging area ID.

(2.3. Interaction with NAS)

Discussion point 8 that “Does the UE NAS need to be aware when the UE isin light RRC connection? please justify your response” seems to implythe possibility of some additional interaction between NAS and AS.

RAN2 agreed to go with Modelling A-2, i.e., RRC Connected-based model,which is beneficial in order to avoid the ECM state mismatch between theMME and the UE, i.e., to stay in ECM Connected. So, it could be assumedthat NAS may just perform ECM Connected procedures as it is today,during the UE in Light Connection. In other words, Light Connection maybe transparent from NAS's perspective. For example, when NAS signallinghappens, AS just initiated RRC Connection Resume procedure to get back“full” RRC Connected state.

Observation 1: As the baseline, Light Connection is transparent fromNAS's perspective.

However, there could be some abnormal case, such as RRC ConnectionResume procedure is failed, i.e., when the UE receives RRC ConnectionReject. It could be seen as “RRC Connection failure” condition fromNAS's perspective, since it's just an error in AS. It seems well alignedwith the information from CT1 that “Since Rel-8, CT1 has below TAUtrigger (in TS 24.301, subclause 5.3.1.3) to re-establish the RRCconnection for the UE in the EMM-CONNECTED mode based on the “RRCConnection failure” indication from the AS layer”.

Considering the information that “CT1 needs more time to study in detailhow to implement the fallback to RRC Connection establishment in thecase of modelling A, but for the above reasons CT1 assume that thefallback will require an explicit interaction between AS and NAS. So itwill probably be more similar to the above TAU trigger or a similartrigger for a service request specified in the current CT1specification.”, it should be avoided to specify additionalfunctionality for similar mechanism, i.e., AS should just consider andinform of “RRC Connection failure” upon reception of RRC ConnectionReject during the transition from Light Connection to RRC Connected.

Proposal 5: RAN2 should agree that AS informs NAS of “RRC Connectionfailure” when the UE fails to get back from Light Connection to “full”RRC Connected.

(2.4. Light Connection Support Indication)

Discussion point 9 was that “Does the eNB need to broadcast a light RRCconnection? please justify your response.”, since it's still FFS that“UE should know whether a UE lightly connected function is supported ornot in the cell.” It could be assumed that the UE in Light Connection is“Performing cell reselection based mobility, the same cell reselectionmechanism in RRC IDLE”, as long as all the eNBs in a network support thereturn from Light Connection to RRC Connected. On the other hand,although it might be up to NW implementation, Rel-13 didn't assume alleNBs in a network supports the new features, so it had the indicationwhether each new feature is allowed to be initiated, e.g., eDRX-Allowedfor eDRX, voiceServiceCauseIndication for VoLTE Establishment Cause,up-CIoT-EPS-Optimisation and cp-CIoT-EPS-Optimisation for RRC ConnectionResume and data over NAS respectively.

There is two possible cases that the UE initiates the RRC ConnectionResume during in Light Connection, i.e., for return to “full” RRCConnected and for RAN paging area update (PAU). The former case may notbe possible if RAN paging area does not include the cells which don'tsupport Light Connection, but the latter case is still problematic. TheUE would initiate a special RRC Connection Resume procedure for PAUwhenever it enters the cell outside of RAN paging area, i.e., “RRCConnection Resume procedure triggered by AS for PAU shall includeadditional indication for PAU”, but still the UE does not know whetherthe cell is acceptable to receive the PAU procedure or not. So, the UEshould be informed in SIB whether the cell supports Light Connection ornot.

Proposal 6: RAN2 should introduce an indication in SIB2 if the cellsupports Light Connection, i.e., if the UE is allowed to send an RRCConnection Resume Request during in Light Connection.

If Proposal 6 is agreeable, the question is how the UE in LightConnection should behave when it enters the cell belonging to such aneNB, since the UE may become unreachable from RAN-initiated paging forMT call and/or may not initiate RRC Connection Resume for MO call.

One of possible solutions could be considered in the cell reselectionprocedure. For example, the UE may prioritize the cell supporting LightConnection, whereby the cell may be determined by e.g., the configuredRAN paging area (with the list of cells) or the SIB indication (Proposal6). This enhancement is expected to avoid the problematic condition asmuch as possible, e.g., in case that only one frequency layer does notsupport Light Connection. So, the UE should be allowed to prioritize thecell supporting Light Connection, in the cell reselection during inLight Connection.

Proposal 7: RAN2 should agree that the UE is allowed to prioritize thecell supporting Light Connection in the cell reselection procedure.

Even if Proposal 7 is applicable, it's still possible that the UE inLight Connection eventually reselects the cell which does not supportLight Connection, e.g., since no cell supporting Light Connection isfound on the UE's location. In this case, the UE in Light Connectionshould autonomously transition to RRC IDLE, and possibly AS informs NASof “RRC Connection failure” to trigger the NAS Recovery, similar to thefailure case discussed in section 2.3. In addition, it should be alsodiscussed when the UE needs to transition to IDLE, i.e., whetherimmediately or only upon MO signalling/data happens or MT accessreceived.

Option 1: Transition to IDLE immediately whenever the UE enters the cellnot supporting Light Connection:

Pros: It's the simplest behaviour, from the UE point of view.

Cons: The opportunity staying in Light Connection may be minimized;

-   -   therefore, the additional signalling, to transition from IDLE to        RRC Connected and also for NAS Recovery, is required whenever        the UE goes through the cell not supporting Light Connection.

Option 2: Transition to IDLE only upon MO/MT call happens in the cellnot supporting Light Connection:

Pros: The UE may continue keeping in Light Connection, unless MO/MT callhappens in the cell not supporting Light Connection.

Cons: The UE in Light Connection needs to monitor the legacy paging,i.e., IMSI or S-TMSI in the paging message, like as in IDLE.

Alternatively, it could be also possible that the UE takes the actionbefore the cell reselection, as follows;

Option 3: Initiate RRC Connection Resume before it reselects a cell notsupporting Light Connection:

Pros: The (anchor) eNB controls the UE's transition of RRC states. It'salso expected to minimize the latency for call re-setup, comparing tothe other options. Additionally, the (anchor) eNB may determine thenecessity of the UE context (i.e., either remove the context or transferthe context).

Cons: The duration in Light Connection may be shortened, similar toOption 1. Some extra standardization efforts may be necessary, e.g., theserving cell may provide the list of neighbouring cells not supportingLight Connection and the UE may need to inform the eNB when it is aboutto reselect to a cell not supporting Light Connection using the RRCResume procedure etc.

Considering the objective of this WI [11], Option 2 is preferable forsignalling reduction.

Proposal 8: RAN2 should agree that the UE in Light Connection shouldtransition to IDLE (and/or AS informs NAS of “RRC Connection failure”)upon MO/MT call happens, when it's in the cell not supporting LightConnection.

Although it's not a problem with Option 3, for Options 1 and 2 it needsto be assumed that some sort of “fallback” mechanism of RAN-initiatedpaging in the NW, e.g., the “anchor eNB” asks the MME to initiate S1PAGING when it notices RAN-initiated paging to the UE is unreachable,since the eNB does not support Light Connection anyway.

Proposal 9: RAN2 should discuss whether the “fallback” mechanism to thelegacy paging is necessary.

FIG. 16 illustrates options of UE behaviour for cell not supportingLight Connection.

(2.5. Paging Occasion (IMSI Mod x))

It was agreed that “The UE ID (IMSI mod x) is used for PO/PF calculationin RAN-based paging”. However, it's still unclear how the eNB knows theIMSI of a specific UE in Light Connection, in order to determine thePF/PO of the UE. Currently it can be known when the eNB receives S1PAGING, i.e., “(Extended) UE Identity Index Value” which is UE_ID, i.e.,IMSI mod 1024 or 4096. However, it's somewhat strange that the eNB hasto ask the MME to send the message, whenever the eNB wants to sendRAN-initiated paging to the UE in Light Connection.

Observation 2: It's still unclear how the eNB determines PF/PO of the UEin Light Connection.

The three alternatives could be considered as follows.

Alternative 1: The eNB gets “UE Identity Index Value” from the MME, whenit makes the UE to enter Light Connection.

Alternative 2: The UE informs the eNB of either its IMSI or UE_ID.

Alternative 3: The eNB comprehends the UE's IMSI within NAS PDU, e.g.,ATTACH REQUEST.

Alternative 1 is similar to the current concept, i.e., “(Extended) UEIdentity Index Value” is managed and provided by the MME. However, it'ssomewhat conflict with the WI objective that “Signalling reduction to CNover S1 interface due to mobility and state transitions by hiding themfrom CN”, and also it needs the additional standardization efforts inthe other WG.

Alternative 2 may be decided within RAN2, but the issue is which messageshould convey the information. Usually, it may be natural to inform ofIMSI or UE_ID when the UE enters Light Connection, i.e., to use“Complete” message, but it's already agreed to go with RRC ConnectionRelease procedure which does not have the response message. Anotherpossibility is to use UE Information procedure or UE Capability Transferprocedure, but it requires that the eNB always Request/Enquiry to getthe information.

Alternative 3 relies on the implementation, so a minimum impact on thespecifications is expected. However, it's unclear if it could beacceptable to assume such an implementation, i.e., cross-layerinteraction.

Although these alternatives have pros and cons, Alternative 2 may bepreferable in order to complete the WI by the deadline.

Proposal 10: RAN2 should agree that the UE may inform the eNB of eitherits IMSI or UE_ID, in order for the eNB to determine the PF/PO.

Proposal 11: RAN2 should discuss which message to be used for theinformation transfer.

Additionally, it's necessary for the “anchor” eNB to transfer the IMSI,UE_ID or “UE Identity Index Value” to the other eNB, when a MT callhappens to the UE in different eNB's coverage, e.g., via “X2 PAGING”.

Observation 3: The IMSI or UE_ID needs to be transferred from the“anchor” eNB to the other eNB during RAN initiated paging process.

(2.6. Awareness of Data Inactivity During RRC Connected)

Since “A UE enters into “lightly connected” by RRC signalling”, theserving cell needs to determine when to trigger the UE to enter LightConnection. One of the possible implementations is for the serving cellto monitor the traffic behaviour and trigger the UE to enter LightConnection due to the UE's inactivity for a period of time. Since thismechanism relies on the expected traffic behaviour, if the estimation ofthe expected traffic is inaccurate then the signalling overhead mayactually increase, e.g., due to frequent transitions between LightConnection and RRC Connected, or the chance to entering Light Connectionis missed. While expected MTC-type traffic can be easily estimated,LTE-type traffic, esp. smartphone's traffic behaviour, may not be aseasy for the NW to predict. Therefore, it may be necessary for the UE toprovide some assistance information since the UE has a betterknowledge/control of its traffic behaviour. So, it's worth consideringwhether the serving cell may configure the UE to provide certainassistance information to allow the eNB to make a better decision totrigger the UE to Light Connection.

Proposal 12: RAN2 should discuss whether the serving cell may configurethe UE to provide assistance information to allow the eNB to make abetter decision on when to trigger the UE to enter Light Connection.

If Proposal 12 is agreeable, the assistance information may have somesimilarity to the existing Power Preference Indicator (PPI) and/or MBMSInterest Indication (MII). With PPI, the UE may inform oflowPowerConsumption when its power consumption is preferred to beoptimized by e.g., longer DRX cycle. The MII was used to inform of theMBMS frequencies of interest and the priority between Unicast and MBMSe.g., when the handover to the frequency is preferred. In this case, theUE may inform the serving cell when it is appropriate for the UE toenter Light Connection; in other words, the UE may send the assistanceinformation when the data transmission/reception has been or will beinactive within a certain duration. The details and necessity of anyadditional assistance are FFS, e.g., UE's expected inactive time.

Proposal 13: RAN2 should consider if the UE should send the assistanceinformation upon data inactivity.

The invention claimed is:
 1. A base station used in a mobilecommunication system, comprising: a controller configured to transmit aresume identifier used for resuming an RRC connection of a userequipment, to the user equipment when causing the user equipment totransition from an RRC connected state to an RRC inactive state in whicha paging area is configured in the user equipment from the base station,transmit to another base station, a paging request message for pagingthe user equipment, the paging request including the resume identifierand information indicating a paging DRX (Discontinuous Reception) cycleof the user equipment, and receive from the another base station, firstinformation indicating that the another base station has failed inpaging the user equipment.
 2. The base station according to claim 1,wherein the controller is configured to receive from the user equipment,assistance information indicating that a preferred RRC state of the userequipment is the RRC inactive state.
 3. An apparatus for controlling abase station comprising: a processor and a memory, the processorconfigured to cause the base station to transmit a resume identifierused for resuming an RRC connection of a user equipment, to the userequipment when causing the user equipment to transition from an RRCconnected state to an RRC inactive state in which a paging area isconfigured in the user equipment from the base station, transmit toanother base station, a paging request message for paging the userequipment, the paging request including the resume identifier andinformation indicating a paging DRX (Discontinuous Reception) cycle ofthe user equipment, and receive from the another base station, firstinformation indicating that the another base station has failed inpaging the user equipment.
 4. A method used in a base station,comprising: transmitting a resume identifier used for resuming an RRCconnection of a user equipment, to the user equipment when causing theuser equipment to transition from an RRC connected state to an RRCinactive state in which a paging area is configured in the userequipment from the base station, transmitting, to another base station,a paging request message for paging the user equipment, the pagingrequest including the resume identifier and information indicating apaging DRX (Discontinuous Reception) cycle of the user equipment, andreceiving, from the another base station, first information indicatingthat the another base station has failed in paging the user equipment.